The prior art takes a number of approaches to making potentially modal diaphragms act like a piston:    1) drive on the nodal line of a chosen mode to suppress that specific mode (usually the lowest mode),    2) drive uniformly over the entire area, such as is the case with an electrostatic or Magnaplanar speaker, or    3) specific, asymmetric arrangements of two drivers, see for example U.S. Pat. No. 4,426,556 of Matsushita.
The BMR teaching of WO2005/101899A aims to balance a modal radiator such that its modes resemble those of the free panel up to a chosen order. It achieves this balance by appropriate selection of the positioning and mass of the drive part of the transducer and of at least one mechanical impedance means, e.g. mass.